SBU’s conclusions

The purpose of this report was to systematically collate scientific literature on dietary advice for obese individuals, or the actual food intake among such people. The following conclusions are limited to links between food and weight, morbidity and mortality among obese individuals (defined as people with a BMI of ≥30 kg/m2 or a waist size of ≥102 cm (men) or ≥88 cm (women). Links with other outcomes are described in the introductions to each subsection of this report. SBU has previously discussed diet for individuals with diabetes. The results for obese individuals and individuals with diabetes generally point in the same direction. This report does not evaluate implementation methods for passing on dietary advice. Nor have we reviewed the scientific literature for links between diet and ill-health among the population in general.

Weight loss in adults. A range of advice on alteration of eating and drinking habits can result in obese individuals losing weight or reducing their waist size. In the short term (six months), advice on strict or moderate low carbohydrate diets is a more effective means of achieving weight loss than advice on low fat diets. In the long term, there are no differences in the effect on weight loss between advice on strict and moderate low carbohydrate diets, low fat diets, high protein diets, Mediterranean diets, diets aimed at achieving a low glycaemic load or diets containing a high percentage of monounsaturated fats. Advice on increasing the intake of dairy products (primarily milk) or reducing the intake of sweet drinks may also lead to weight loss.

Weight loss in children and young people. Advice on increasing the intake of dairy products (primarily milk) in the event of energy restriction may lead to weight loss among obese children and young people. There is insufficient scientific data to assess whether other dietary advice which is effective for obese adults is also effective for people under 18.

Maintaining reduced weight. When obese individuals have lost weight, they can maintain their weight more effectively with advice on low fat diets with a low glycaemic index and/or high protein content rather than low fat diets with a high glycaemic index and/or low protein content. There is no data available to assess whether advice on low carbohydrate diets and Mediterranean diets, for example, is effective to prevent weight increase after weight loss.

Onset of cardiovascular disease or death caused by cardiovascular disease. For obese individuals, intensive advice on Mediterranean diets (with extra olive oil or nuts and almonds) leads to a reduced risk of onset of cardiovascular disease or death caused by cardiovascular disease, compared with advice on low fat diets. People who drink a lot of coffee also have a lower mortality rate, regardless of cause.

Onset of type 2 diabetes. Obese individuals run a greatly increased risk of diabetes. The risk of falling ill with diabetes is lower among people who drink alcohol and people who drink a lot of coffee, while it is higher among people who drink sweet drinks. However, advice on low fat diets does not lead to a reduced risk of falling ill with diabetes compared with advice on diets with a standard fat content among obese post-menopausal women.

Knowledge gaps. It is not possible to assess whether other types of diet or drink are of significance to morbidity or mortality among obese individuals. There is insufficient scientific data. There is likewise insufficient data to assess the effect of the foods studied – fruit, vegetables, wholemeal products, legumes, potatoes, soya products, meat and processed meats – on mortality, morbidity or weight loss among obese individuals.

SBU Assessment
presents a comprehensive, systematic assessment of available scientific evidence. The certainty of the evidence for each finding is systematically reviewed and graded. Full assessments include economic, social, and ethical impact analyses.

SBU assessments are performed by a team of leading professional practitioners and academics, patient/user representatives and SBU staff. Prior to approval and publication, assessments are reviewed by independent experts, SBU’s Scientific Advisory Committees and Board of Directors.